Unless you have the specific package version's
.deb file downloaded, you can only get what's available in the Ubuntu Repositories (or third party repos or PPAs configured on your system), and usually they only keep the Latest Copy of the package available in the Ubuntu repository itself at any given time.
This is the
rmadison output for
rdiff-backup in all current releases:
$ rmadison -u ubuntu rdiff-backup
rdiff-backup | 1.2.8-7 | precise/universe | source, amd64, armel, armhf, i386, powerpc
rdiff-backup | 1.2.8-7 | trusty/universe | source, amd64, arm64, armhf, i386, powerpc, ppc64el
rdiff-backup | 1.2.8-7 | xenial/universe | source, amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
rdiff-backup | 1.2.8-7 | bionic/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
rdiff-backup | 2.0.0-1 | focal/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
rdiff-backup | 2.0.5-1 | groovy/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
Therefore, if you are using any release before 20.04, you are stuck with either Nothing (because EOL) or 1.2.8-7. Any release after and including 20.04, you're stuck with 2.0.0 or later.
If you want another version, you'll have to go digging elsewhere because older versions would be ancient, and any OLDER version than 1.2.8-7 (while you can go hunting to TRY and find the
.deb for it in older pages and such, or try and compile the older versions yourselves, both of which are nasty options because you're going into 'untrusted sources' to get older versions.
Unless you explicitly need the older versions than 1.2.8-7, you should stick with the version of the package in the repos for your specific release, and not try and install a specific other version - which
apt can't do if there are no other versions available in your repository sources for your codenamed release (Bionic for 18.04, Focal for 20.04, for example) (Oh, and there isn't another version available for any of the releases at this time per
rmadison, except for the ones it lists per-codename.).
Also, keep in mind that
1.2.8-7 is version 1.2.8 but with package level revisions. ALSO keep in mind that at the time of 1.2.8 it had not migrated to Python 3 support - as a result, 1.2.8 may not function on newer versions of Ubuntu because of Python 2 package deprecation and drops in Debian which trickled down to Ubuntu 20.04 and later.